1. Field of the Invention
The present invention relates to a television tuner integrated circuit including a high frequency circuit for a first band and a high frequency circuit for a second band, and to a television tuner including the integrated circuit.
2. Description of the Related Art
Conventionally, in a television tuner which receives a television signal of a VHF (Very High Frequency) band and a television signal of a UHF (Ultra High Frequency) band, an RF amplifier is provided to separately perform high frequency amplification on the television signal of the VHF band and the television signal of the UHF band. As an example of the RF amplifier, an RF amplifier has been proposed which includes a pair of dual gate FET amplifiers for individually amplifying the television signal of the VHF band and the television signal of the UHF band and a change-over switch for switching between the dual gate FETs to be actuated in accordance with the VHF band and the UHF band (see Japanese Unexamined Patent Application Publication No. 2002-368639, for example).
FIG. 3 is a configuration diagram of a television tuner including an RF amplifier unit capable of switching between dual gate FETs. A received television signal is input to a UHF antenna tuning circuit 101 and a VHF antenna tuning circuit 102 from a not-illustrated antenna. Output terminals of the UHF antenna tuning circuit 101 and the VHF antenna tuning circuit 102 are connected to an RF amplifier 200. A UHF output terminal 200a and a VHF output terminal 200b of the RF amplifier 200 are connected to a UHF multiple-tuning circuit 110 and a VHF multiple-tuning circuit 120, respectively. An output terminal of the UHF multiple-tuning circuit 110 and an output terminal of the VHF multiple-tuning circuit 120 are connected to UHF input terminals 301a and 301b and a VHF input terminal 302a of a tuner integrated circuit 300, respectively.
The RF amplifier 200 includes a UHF dual gate FET 201 having a first gate connected to the output terminal of the UHF antenna tuning circuit 101, a VHF dual gate FET 202 having a first gate connected to the output terminal of the VHF antenna tuning circuit 102, and a FET 203 for switching between the two dual gate FETs 201 and 202. A source of each of the UHF dual gate FET 201 and the VHF dual gate FET 202 is connected to the ground, and a drain of the UHF dual gate FET 201 and a drain of the VHF dual gate FET 202 are connected to the UHF output terminal 200a and the VHF output terminal 200b, respectively, to which a power supply voltage B is applied. Each of the UHF dual gate FET 201 and the VHF dual gate FET 202 is applied with an AGC voltage VAGC at a second gate thereof. The FET 203 has a source connected to the ground, a gate connected to the first gate of the UHF dual gate FET 201 via a resistor 204, and a drain connected to the first gate of the VHF dual gate FET 202 via a resistor 205 and also connected to the UHF output terminal 200a via a resistor 206. Details of the circuit configuration of the RF amplifier 200 are disclosed in the above-described publication.
The UHF output terminal 200a of the RF amplifier 200 is applied with the power supply voltage B via a series circuit formed by a feed resistor 111 and a choke inductor 112. A primary circuit of the UHF multiple-tuning circuit 110 is formed by a varactor diode 113 and an inductor 114. An intermediate connection point between the anode of the varactor diode 113 and the inductor 114, which is a high potential point, is coupled to the drain of the UHF dual gate FET 201 via a coupling capacitor 115. The cathode of the varactor diode 113 is grounded in terms of high frequency, and the other terminal of the inductor 114 is also grounded. Further, the cathode of the varactor diode 113 is applied with a tuning voltage. The primary circuit of the UHF multiple-tuning circuit 110 is M-coupled to a secondary circuit 116 of the UHF multiple-tuning circuit 110. The secondary circuit 116 is similar in configuration to the primary circuit.
The VHF output terminal 200b of the RF amplifier 200 is connected to a primary circuit of the VHF multiple-tuning circuit 120. The primary circuit of the VHF multiple-tuning circuit 120 is formed by a varactor diode 121 and inductors 122 and 123. An intermediate connection point between the cathode of the varactor diode 121 and the inductor 122, which is a high potential point, is coupled to the drain of the VHF dual gate FET 202. The anode of the varactor diode 121 is grounded, and the cathode of the varactor diode 121 is applied with a tuning voltage. The other terminal of the inductor 123 is grounded via a coupling capacitor 126. Further, the other terminal of the inductor 123 is applied with the power supply voltage B. To switch between a VHF low band and a VHF high band, an intermediate connection point between the inductors 122 and 123 is connected to the anode of a diode 124. The cathode of the diode 124 is grounded in terms of high frequency via a coupling capacitor 127. A secondary circuit 125 of the VHF multiple-tuning circuit 120 is M-coupled to the primary circuit of the VHF multiple-tuning circuit 120. The secondary circuit 125 is similar in configuration to the primary circuit.
The tuner integrated circuit 300 includes a UHF RF circuit 301 formed by, for example, a mixing circuit for performing frequency conversion on a received television signal of the UHF band, and a VHF RF circuit 302 formed by, for example, a mixing circuit for performing frequency conversion on a received television signal of the VHF band. The UHF RF circuit 301 is connected to the UHF input terminals 301a and 301b applied with a balanced output from the secondary circuit 116 of the UHF multiple-tuning circuit 110. The VHF RF circuit 302 is connected to the VHF input terminal 302a applied with an unbalanced output from the secondary circuit 125 of the VHF multiple-tuning circuit 120. Further, an external terminal 310 is applied with a direct current bias voltage Vcc. The external terminal 310 is connected to the UHF input terminal 301b via between a collector and an emitter of a transistor 311. A base of the transistor 311 is applied with a UHF band selection signal (BS4), which becomes high level when the UHF band is selected. A resistor 312 is provided between the emitter of the transistor 311 and the UHF input terminal 301b. A UHF/VHF switch line 314 is provided between the UHF input terminal 301b, which is applied with the direct current bias voltage Vcc via the transistor 311, and a UHF input terminal 200c of the RF amplifier 200. Further, the external terminal 310 applied with the direct current bias voltage Vcc is connected to an external terminal 322 via a resistor 321. The external terminal 322 is connected to the cathode of the diode 124 via a VHF multiple-tuning circuit switch line 324. The VHF multiple-tuning circuit switch line 324 is grounded via between a collector and an emitter of a transistor 323 provided in the tuner integrated circuit 300. A base of the transistor 323 is applied with a VHF high band selection signal (BS2), which becomes high level when the VHF high band is selected, and the UHF band selection signal (BS4).
Description will now be made of a UHF/VHF switching operation performed in the above-described television tuner. When the UHF band is selected, the transistor 311 is turned on, and the direct current bias voltage Vcc is applied to the UHF input terminal 200c of the RF amplifier 200 via the UHF/VHF switch line 314. Thereby, the UHF dual gate FET 201 is applied with the direct current bias voltage Vcc at the first gate thereof, and becomes capable of performing an amplifying operation. Meanwhile, the gate of the FET 203 is also applied with the direct current bias voltage Vcc, and the first gate of the VHF dual gate FET 202 is grounded via the FET 203. Accordingly, the first gate of the VHF dual gate FET 202 is applied with a voltage of 0 V, and becomes non-operational.
On the other hand, when the VHF band is selected, the UHF band selection signal (BS4) becomes low level. Thus, the transistor 311 is turned off, and the UHF input terminal 200c of the RF amplifier 200 is applied with the voltage of 0 V from the UHF/VHF switch line 314. Accordingly, the UHF dual gate FET 201 is applied with the voltage of 0 V at the first gate thereof, and becomes non-operational. Meanwhile, with the FET 203 turned off, the VHF dual gate FET 202 is applied with the power supply voltage B at the first gate thereof, and becomes capable of performing an amplifying operation. Further, when a high band of the VHF band is selected (and when the UHF band is selected), the VHF high band selection signal (BS2) becomes high level, and the cathode of the diode 124 is grounded. Accordingly, the diode 124 becomes conductive. Thus, the primary circuit of the VHF multiple-tuning circuit 120 resonates at a frequency in accordance with the inductor 122, and the VHF high band is selected. Meanwhile, when a low band of the VHF band is selected, the VHF high band selection signal (BS2) and the UHF band selection signal (BS4) both become low level, and the transistor 323 is turned off. Accordingly, the diode 124 becomes nonconductive. Thus, the primary circuit of the VHF multiple-tuning circuit 120 resonates at a frequency in accordance with the two series-connected inductors 122 and 123, and the VHF low band is selected. In the above-described manner, the diode 124 is turned on and off to switch between the VHF high band and the VHF low band.
In the tuner integrated circuit 300, however, the UHF input terminal 301a (301b) of the UHF RF circuit 301 and the VHF input terminal 302a of the VHF RF circuit 302 are disposed to be adjacent to each other. Thus, a parasitic capacitance Co exists between the input terminals. Therefore, when a signal of the UHF band is received, a VHF signal (VHF high band) corresponding to a half of the frequency of the UHF band enters the UHF RF circuit 301 via the parasitic capacitance Co to act as an interference signal.